The present invention relates to a display apparatus such as a computer terminal, and more particularly, to a display apparatus that is capable of displaying video signals having different deflection frequencies of horizontal deflection signal provided by various kinds of computers and image producing apparatus.
At present, display apparatus such as computer terminals receive image signals (video signals) of non-standard specifications from computers or the like, and the video signals to be displayed have various scanning frequencies, image display periods, image display positions and image flyback periods. On this account, there is a trend of using a xe2x80x9cmulti-scan display apparatusxe2x80x9d that performs, as a single unit, proper image display for various image signals (video signals).
Among display apparatus of this type, there are many prior art publications pertinent to the reformation for multi-scanning, such as one, as described in Japanese Patent Publication No. 61-8628, originally published as Japanese Patent Application Laid-open No. 53-57914, or the like, which is intended to deal with many scanning frequencies of video signals by making the horizontal oscillation frequency that is equal to the horizontal deflection frequency of the horizontal deflection circuit variable to match with the sync signal frequency of a video signal received by the display apparatus by use of a sync oscillation device, and another one, as described in Japanese Patent Application Laid-open No. JP-A-61-100078 or the like, which is intended to adjust the display position on the display screen in coping with the display specifications by shifting the phase of the sync signal in the video signal by use of a monostable multivibrator in a display position modification circuit.
Furthermore, Japanese Patent Application Laid-open No. JP-A-3-73994 discloses a multi-frequency operative CRT display apparatus having a detection circuit which produces a detection signal if the frequency of the horizontal sync signal received by the display apparatus is lower than the preset frequency and a conversion circuit which doubles the deflection frequency of the horizontal deflection circuit and the frequency of the video signal received by the display apparatus in response to the detection signal.
However, this display apparatus, which implements the frequency doubling conversion if the input horizontal sync signal frequency is lower than the predetermined value, achieves the intended effect only for the horizontal sync signal of a particular frequency by repetitive read out of the same scanning line, thereby repeating the content thereof, and cannot deal with signal frequencies in a wide range.
U.S. Pat. No. 5,229,853 discloses a system for converting a video signal from a first format to a second format and is directed to converting from a higher frequency to a lower frequency with a reduction in the number of scanning lines and a reduction in resolution.
The above-mentioned conventional apparatus use many active devices in the deflection circuit in order to deal with the extensive video signal specifications and need a large scale circuitry in order to maintain the operational stability and reliability, leaving problems in achieving the easy fabrication and adjustment of the display apparatus.
Recently, video signals have their scanning frequencies shifted toward the higher region, and consequently the video signal specifications which need to be covered by display apparatus are expanding. Particularly, from the viewpoint of the scanning frequency of video signal, it is desirable to meet a horizontal scanning frequency range from 15.75 kHz of NTSC signals to about 90 kHz for high resolution pictures and CAD/CAM applications.
It is not feasible for the above-mentioned conventional apparatus intact to deal with signals of such a wide range of frequency, but they will be involved in an intricate circuitry and increased number of component parts for the switching control of operational parameters and in an increased circuit scale for retaining the operational reliability. Moreover, it becomes difficult to keep the performance against the deterioration of picture quality and the distortion of image for each video signal. As a result, the number of adjustment points will increase and manufacturing cost will rise.
Accordingly, an object of the present invention is to provide a display apparatus which can deal with video signals having a variety of scanning frequencies and different resolutions, with the enhancement being made for the performance, reliability and operational stability while suppressing an increase in manufacturing cost.
Another object of the present invention is to provide a display apparatus for receiving video signals which differ in scanning frequencies or resolutions and for converting a received video signal to be within prescribed higher ranges without repetitive readout of the same scanning line.
In order to solve the foregoing problems, a display apparatus is formed of a frequency/resolution conversion unit for converting the scanning frequency and resolution of a video signal and a multi-scan display unit having its operational frequency range suppressed.
According to a feature of the present invention, the frequency/resolution conversion means operates to raise the horizontal scanning frequency or vertical scanning frequency of a video signal received by the display apparatus up to a frequency within a prescribed range, and the multi-scan display unit operates to display the video signal, which has been rendered the frequency conversion by the process of the frequency/resolution conversion unit, accurately at the prescribed position of the screen.
Other and further objects and features of the invention will appear fully from the following description.